Contents

Panic ensues when a museum exhibit is stolen. K9, Starkey, Darius and Jorjie are on the trail of the culprit. They find a giant menace hiding in London's abandoned sewers. Is the alien the threat or is Drake the real evil? Starkey and K9 face annihilation as they try to rescue the alien's hatchlings before a bomb destroys them all.

In a museum, K9, Jorjie and Starkey stroll through a virtual realitySherwood Forest. They find a notice stating, "Here stood the Major Oak, sanctuary to the outlaw Robin Hood and his band of Merry Men." Jorjie tells Starkey she thought she would marry Robin Hood when she was a kid. When Starkey notes that both Robin Hood and the Major Oak are long gone now, a virtual reality simulation of Robin Hood comes out of the trees and contradicts him. He says the Major Oak lives in the museum and welcomes them to Sherwood Forest. K9 says he should be shorter. Robin is shocked at this "sorcery." Jorjie is surprised K9 has met the real Robin Hood. When Starkey asks Robin Hood about his tights, he tells him it was "chilly" in his day. K9 confirms that Britain was three degrees cooler in the Middle Ages; Robin notes that pollution has affected the planet and the oaks and silver birches have been cut down or died out. Robin Hood starts to glitch and loop and K9 detects an alien threat. They turn off the virtual reality forest as the patrons flee.

A Department officer says a signal of alien origin is causing magnetic interference in Sector SE14 Delta. They patch into a CCPC's vision and June sees the CCPCs malfunctioning. She orders Inspector Drake to the scene. The curator insists the oak tree hasn't been misplaced. K9 detects an unidentified alien species and decides to investigate. The Major Oak has been stolen. June arrives and stops the CCPCs, who think the curator is a suspect, with a command override. They find a hard, alien substance where the tree used to be. K9 scans it. It could be vomit or snot, but he needs to examine a sample. He blasts off a chunk. Starkey picks it up and they run off.

At the mansion, Gryffen listens to news of the theft. K9, Starkey and Jorjie arrive to show him the alien substance, which K9 says is emitting a high-pitched frequency. As June examines where the Major Oak was displayed, Drake arrives with orders from Lomax. The aliens have been classed as an A-level threat, "which means it's time to let the big boys play". June claims this is not his jurisdiction, but Drake picks up a piece of the alien substance. Gryffen tests the substance's magnetic field, confirming it made the CCPCs malfunction. K9 is not affected. Unlike the CCPCs, his electromagnetic shielding is impenetrable. Darius arrives, wondering if the museum is offering a reward, disgusting Jorjie with his selfishness. Gryffen asks Darius if the post office had his invitation for the Annual Bournedecker Awards for Scientific Excellence. When Starkey wonders if he'll leave the house, he says he won't. Gryffen says that he sees it as a tradition that every year he gets an invitation and every year he doesn't show up. Darius suggests the postman lost it. In fact, the postman is trapped underground by the alien substance.

Gryffen completes the analysis of the substance, which is actually paper-mache. Whatever they're dealing with is genetically altered. When Starkey wonders why it chose the Major Oak, Gryffen states that many species target one species. Considering what it did to recycled paper, he hates to think what it could do with the Oak. K9 suggests that he return to the scene of the crime and scan for a trail. Jorjie says the museum is probably patrolled by the Department, but Starkey says they know where to find the thieves: in the sewers. K9, Starkey, Jorjie and Darius search the sewers. Gryffen detects interference while monitoring K9, and asks him if he's all right. K9 replies that it was just a spike in the electromagnetic interference. He finds oak leaves on the floor.

June contacts Gryffen to tell him the alien substance has stumped the Department scientists. Gryffen says they're "slow-witted" and he has already unravelled the sample's mysteries. Gryffen says that the thieves won't make ransom demands as they aren't looking for trade and already have what they came for. The team finds slime on the sewer walls. Darius, Jorjie and K9 move on. As Starkey takes a closer look, he finds slimy oak leaves, hears a creature growling and decides to move on. Gryffen monitors K9, who seems to be confused. K9 confirms he is experiencing the same electromagnetic interference created by the substance from the museum and stares into space. K9 tells Darius the interference is getting worse. Darius looks around and finds no sign of Starkey. As Starkey searches, his light flickers and he calls out for his friends. The creature's growls grow louder as the light dims. When it turns off, the alien creeps behind Starkey.

Darius and Jorjie hear something fall. The creature drags Starkey to its lair. Jorjie asks if K9 is sure he knows where he's going. He tells her that he does. Gryffen's videolink with K9 starts to break up, but K9 tells him everything is all right on his end. Gryffen knows something is affecting K9's circuits. He compares the high-pitched frequency of the alien substance to a dog whistle. It is sending out a distress signal. He tries to tell K9 but communications are scrambled. Starkey, now trapped in the alien substance, warns the alien what Jorjie will do once she finds out what they've done to the Major Oak. The postman is trapped next to Starkey and hopes that Jorjie is a "super soldier with a couple of bazookas". He gives his name as Patrick. Starkey gets his dog whistle out and calls for K9, but this only gains the attention of the alien creature.

The magnetic interference weakens and Gryffen can talk to K9 again. K9 notes that the magnetic interference matches the pitch of a dog whistle. Gryffen explains to Darius that K9 was confused as he's been following an alien distress signal. K9 will consider the alien hostile until he can confirm Starkey is safe. As Starkey whistles again, Patrick and he are released and fall to the ground, where they sees alien eggs. Starkey continues to whistle as K9 races to find him. K9, Jorjie and Darius find Starkey, but before K9 can neutralise the creature, Starkey shows him the eggs; the creature is a mother, and its lair is a hatchery. The creature doesn't want to attack or eat anyone. She's just protecting her babies. Gryffen tells K9 not attack the creature, known as a Centuripede. She's here to build a pod for eggs, or as K9 and Darius put it, an "interstellar baby carriage" or "a spaceship made out of wood and centipede spit". She specifically needs oak wood. Centuripedes have bred in oak forests for thousands of years, but since there are no more oak trees this may be the last spawning of the Centuripede on Earth.

June contacts Gryffen, who reports the situation to her. She says that both the alien and K9 are "history" if Drake catches them. Drake sets a gas bomb to detonate in four minutes. Darius runs with Patrick, who is carrying Jorjie, who doesn't want to leave the aliens. K9 insists that Starkey leaves, but he wants to help save the aliens. K9 tries to keep the gas out with a photon spread, but he can't guarantee he can give the creature enough time. Starkey decides to "make time". As K9 holds off the vapour, Starkey helps the Centuripede put the eggs in the pod. As the others make their way out, K9's power depletes. As the others climb up the ladder, K9 uses his maximum power. Darius is the last to climb up, rummaging through Patrick's postbag to find Gryffen's invitation. As Jorjie, Darius and Patrick get out of the sewer, they see no sign of Starkey or K9. K9 can't hold his power much longer, but Starkey places the final egg and the pod rises. The mother collapses and appears to be dead. K9's power is exhausted. Starkey goes to the mother, whose body cracks into white light, knocking him down. The mother transforms into a butterfly-like creature.

The bomb explodes. Darius says there is no chance they could have survived. They spot the mother and her babies flying up into their pod. Darius and Jorjie believe Starkey and K9 dead, but Starkey, who is with K9 behind the others, says how beautiful it was. Starkey tells them the creatures gave them a lift. Starkey shows Jorjie a "token of appreciation" from the creatures: three acorns stashed in their nest from Robin Hood's time. Darius shows Gryffen his invitation on his monitor and discusses his finder's fee. Patrick protests that Darius can't charge people for their mail, but Darius snatches the envelope and runs away while Starkey and Jorjie laugh. The Department's propaganda messages reappear in the sky.